The present invention relates to an in-band audio multifrequency (MF) signal receiver and, more particularly, a digital pushbutton signalling receiver (hereunder referred to as a digital PB receiver) for use in a terminal device for a telephone exchange or the like system.
A digital PB receiver functioning as an in-band MF signal receiver has several advantages over an analog PB receiver in stability and precision over a long period of time under various environmental conditions such as temperature variation, aging variation, etc. In such a PB receiver, two predetermined frequencies, one of which is selected out of a lower group of frequencies (697 Hz, 770 Hz, 852 Hz, and 941 Hz), and the other of which is selected out of a higher group of frequencies (1,209 Hz, 1,336 Hz, 1,477 Hz, and 1,633 Hz), are allotted for the so-called push button telephone signals (PB signals) representing an office number and a subscriber number of a called party. Of these frequencies, 1,633 Hz is not currently used but retained for future use. Besides, dial tones of about 400 Hz may be additionally used for line connection information between said office and a subscriber of a calling party. For this reason, such a PB receiver is required to correctly detect the two desired frequencies even with any environment set, irrespective of the presence or absence of said dial tones.
A digital PB receiver employing digital filters is proposed in FIG. 1 of an article by Shiro Kikuchi et al, entitled "A Study on Digital Pushbutton Signaling Receiver", The Transactions of the IECE of Japan, Vol. E61, No. 8, August issue, 1978, pp. 656-657 (Reference 1). Since an audio frequency band signal containing PB signals is usually sampled at 8 KHz and converted into a digital quantity having a predetermined number of bits in a TDM (time division multiplex) telephone system, the proposed PB receiver of Reference 1 should have all its digital filters process the sampled signal at 8 KHz.
Generally, the number of arithmetic operations performed by each digital filter is proportional to the sampling frequency for each signal processed in the digital filter. Accordingly, the number of arithmetic operations required by a device using digital filters can be reduced by lowering the sampling frequency without losing needed information.
An object of the present invention, therefore, is to provide a digital PB receiver which is capable of detecting each PB signal unevenly distributed in a lowered frequency area within an audio frequency range of 0.about.4 KHz with a smaller number of arithmetic operations by digital processing lowering the PB signal sampling frequencies.
Another object of the invention is to provide a simplified, inexpensive digital PB receiver with less power consumption.